Typically, Sn—Pb-based lead solder has been used as the most effective binding material for electronic devices for a long time. However, when an electronic device using a solder is discarded, lead (Pb) contained in the solder is eluted by acid rain thereby polluting groundwater. Furthermore, such component has been identified as an environmental pollutant because it harms the human body, for example, deteriorating intelligence and reproductive function, when absorbed by the human body. Such lead solder is particularly utilized as a binder for mounting small electronic parts such as semiconductor chips or resistor chips on a printed circuit board.
In particular, the use of lead (Pb)-containing products is strictly limited, and Sn—Pb solder is being replaced with lead-free solder. A variety of restrictions for prohibiting the use of Pb in micro-electronic devices have been imposed. Thus, Sn—Pb solder has to be replaced with Pb-free Sn—Ag solder to develop an environmentally friendly lead-free solder. For this reason, in recent times, when solder alloy is manufactured, the use of lead is restricted or excluded, whereby many attempts have been made to develop an environmentally friendly lead-free solder composition.
Technology relevant to the lead-free solder is proposed in Korean Patent Nos. 0209241 and 0814977.
As disclosed in Korean Patent Nos. 0209241 and 0814977, a lead-free solder composition, and a high-temperature system lead-free solder composition and an electronic device and a printed circuit board using the same, are briefly described below.
The lead-free solder composition comprising tin (Sn), silver (Ag), bismuth (bi) and indium (In), which is disclosed in Korean Patent No. 0209241 (Prior Art 1), is composed of 82 wt % to 93 wt % of tin (Sn), 2 wt % of silver (Ag), 3 wt % to 10 wt % of bismuth (Bi), and 2 wt % to 6 wt % of indium (In).
However, indium (In) needed to form the lead-free solder composition of Prior Art 1 is expensive, and a solder containing bismuth (Bi) may become brittle due to deteriorating ductility with increasing amount of bismuth (Bi).
FIG. 1 is a mimetic diagram schematically showing the principle of inhibiting the generated amount of an oxide of a lead-free solder composition in Korean Patent No. 0814977 (Prior Art 2). As shown in FIG. 1, the high-temperature system lead-free solder composition of Prior Art 2 comprises 2 wt % to 5 wt % of copper, 0.001 wt % to 1.0 wt % of nickel, 0.001 wt % to less than 0.05 wt % of silicon, 0.001 wt % to 0.2 wt % of phosphorus, 0.001 wt % to less than 0.01 wt % of cobalt, and the remainder of tin.
However, the high-temperature system lead-free solder composition of Prior Art 2 also needs indium which is expensive and thus, there is a problem in that the use of indium is limited.
Although not shown in the drawing, recently attention has been focused on the development of solders comprising Sn, Ag, Bi, Cu, In, and Zn, particularly a composition composed of Sn, Ag and Cu. However, the aforementioned respective lead-free solders have various problems. For example, Zn is sensitive to oxidation and resulting decrease in solderability. Solder including Bi is decreased in ductility with an increase in Bi content, thus causing brittleness. Sn—Cu solder is inexpensive, but has very poor wettability. Solder containing In is expensive. Solder containing Ag readily forms a coarse needle-shaped intermetallic compound Ag3Sn, thus decreasing solderability and joint strength. Consequently, not only solder including lead-free solder, but also the aforementioned disadvantages (especially wettability) have to be minimized.
Moreover, Sn-0.7Cu, Sn-3.5Ag, and SAC305 solder (96.5 wt % Sn-3.0 wt % Ag-0.5 wt % Cu) microstructures include a dendritic phase and a eutectic phase composed of Sn, Ag3Sn, and Cu6Sn5. However, the shape and structure of Ag3Sn and Cu6Sn5 are regarded as important in terms of reliability of the solder. If the amount of Ag is greater than 2 wt % in the matrix, a coarse plate of Ag3Sn may be formed, undesirably worsening soldering properties. On the other hand, if the amount of Ag in the Sn—Ag alloy is less than 2 wt %, a liquid temperature may be increased, a coexistence region of a liquid phase and a solid phase is enlarged, and the strength of the joint may decrease. Hence, the growth of intermetallic compounds has to be inhibited using a ceramic nanopowder-reinforcing agent as an additive.